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Syntactic Categories for Dependent Type Theory: Sketching and Adequacy

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Syntactic Categories for Dependent Type Theory: Sketching and Adequacy Syntactic categories for dependent type theory: sketching and adequacy Daniel Gratzer Jonathan Sterling March 22, 2021 Abstract We argue that locally Cartesian closed categories form a suitable doctrine for defining dependent type theories, including non-extensional ones. Using the theory of sketches [KPT99], one may define syntactic categories for type theories in a style that resembles the use of Martin-Löf’s Logical Framework [NPS90], following the “judgments as types” principle [HHP93; Mar87]. The concentration of type theories into their locally Cartesian closed categories of judgments is particularly convenient for proving syntactic metatheorems by semantic means (canonicity, normalization, etc.). Perhaps surprisingly, the notion of a context plays no role in the definitions of type theories in this sense, but the structure of a class of display maps can be imposed on a theory post facto wherever needed, as advocated by the Edinburgh school and realized by the %worlds declarations of the Twelf proof assistant [HHP93; PS99; HL07]. Uemura [Uem19] has proposed representable map categories together with a stratified logical framework for similar purposes. The stratification in Uemura’s framework restricts the use of dependent products to be strictly positive, in contrast to the tradition of Martin-Löf’s logical framework [Mar87; NPS90] and Schroeder-Heister’s analysis of higher-level deductions [Sch87]. We prove a semantic adequacy result for locally Cartesian closed categories relative to Uemura’s representable map categories: if a theory is definable in the framework of Uemura, the locally Cartesian closed category that it generates is a 2-conservative (i.e. fully faithful) extension of its syntactic representable map category. On this basis, we argue for the use of locally Cartesian closed categories as a simpler alternative to Uemura’s representable map categories. 1 Introduction (1·1) What kind of objects are dependent type theories and their models? Unfortu- nately there are many possible answers to this question: 1) Comprehension categories [Jac99] express the structure of a category of contexts equipped with separate notions of type and term, connected by a context ex- tension operation. Sometimes a comprehension category is “split” (modeling strictly associative substitution). 2) Categories with attributes [Car78] are full split comprehension categories; fullness means that the notion of an element is derived from the context extension. 1 3) Categories with families [Dyb96] express the notion of type, term, and context extension as a special kind of universe in the category of presheaves over a category of contexts. These are easily seen to be equivalent to categories with attributes, but they are arguably more type theoretic in style. Awodey [Awo18b] and Fiore [Fio12] have independently reformulated categories with families in terms of representable natural transformations; Awodey has coined the name natural model for this formulation. 4) Contextual categories [Car78] or C-systems [Voe15] are categories with attributes together with a structure that equips each context with a “length”, reflecting the inductive generation of contexts in some presentations of the raw syntax of type theory. 5) Display map categories [Tay86; Tay99] or clans [Joy17] express the data of a category of contexts equipped with a class of “display maps” that generalize context extensions. (1·2) The notion of a contextual category or a C-system is not as useful as it might at first seem: contextual categories differ from categories with attributes onlyby elevating to the status of a definition the incidental aspect of certain raw syntax presentations of type theory that contexts have a length. This influence of (raw) syntax on semantics has so far imparted no practical leverage, even when proving metatheorems of a syntactical nature: indeed, no theorem of dependent type theory could ever have depended on the fact that not every context is of length one. Categories with attributes and natural models are in essence the same notion. Comprehension categories on the other hand occupy an awkward position: full split comprehension categories are the same as categories with attributes and natural models, and full non-split comprehension categories are the same as clans. From our perspective, the canonical notions among the above are therefore clans and natural models, representing the weak and strict notions of dependent type theory respectively. (1·3) We have enumerated a number of scientific hypotheses as to what a model of type theory ought to be; aside from clans, however, (1·1) does not pose a definition of the syntactic/classifying category of a given type theory in the sense of functorial semantics [Law63]. Recently Uemura [Uem19] has proposed representable map cate- gories to serve as the syntactic categories that classify the natural models of a given type theory, which we discuss below in Section 1.1. 1.1 Uemura’s representable map categories (1.1·1) A representable map category is a finitely complete category T equipped with a pullback stable subcategory Trep ⊆ T of “representable maps” such that T con- tains dependent products along representable maps. Representable maps correspond roughly to display maps, and an object Γ: T such that Γ 1T is representable can be thought of as a context. An arbitrary (non-representable) object of T stands for a judgment — something that will be taken to a not necessarily representable presheaf in the natural model semantics. (1.1·2) While a clan expresses the categorical structure of the category of contexts of a given type theory, a representable map category attempts to express the categorical structure of its category of judgments. In particular, while a clan need not be finitely 2 complete (the diagonal is not a display map except in extensional type theories), unrestricted pullback in representable map categories corresponds to the fact that type theory has judgmental equality. Likewise, the presence of (non-representable) dependent products along representable maps corresponds exactly to the hypothetico- general judgment of dependent type theory in the sense of Martin-Löf [Mar96]. (1.1·3) The fact that dependent products need exist only along representable maps corresponds to the way that dependent type theory is conventionally presented using hypothetical judgments of one level only (corresponding by transpose to context extension). This realistic stratification, however, is not at all forced: Martin-Löf himself has promoted a presentation of the syntax of dependent type theory that supports hypothetical judgments of arbitrary level [NPS90; Mar87; Sch87]. One commonly cited example of the use of higher-level judgments is to present dependent products in terms of a “fun-split” operator [NPS90], but this example is not so convincing considering that the presentation is strictly isomorphic to one not involving a higher-level judgment.1 A more convincing example is furnished by the W-type, whose elimination rule apparently cannot even be written down without higher-level hypothetical judgments in the absence of dependent product types. 1.2 Locally Cartesian closed categories (1.2·1) If Uemura’s notion of representable map category aims to capture the restric- tion of a category of judgments to contain just the hypothetical judgments tracked by context extensions, it is reasonable to consider the categorical structure of judgments absent such a stratification. Of course, as soon as we have both judgmental equality and unrestricted hypothetical judgment, the category of judgments is nothing less than a locally Cartesian closed category. Such a syntactic category may be equipped with a class of display maps, but this class is no longer intertwined with the definition of the syntactic category. In contrast, one cannot even write down the closure of the type theory qua Uemura [Uem19] under (e.g.) function types unless certain maps are representable. (1.2·2) Because the category of judgments of a given type theory can be defined independently of the structure of a class of display maps / notion of context, it is in fact reasonable to avoid imposing such a structure until it is needed. The need for a notion of context arises in several situations: 1) Tautologically, one needs a notion of context when defining a formal “gammas and turnstiles” presentation of a given type theory. 2) To correctly state results like decidability of judgmental equality, one also needs a notion of context: judgmental equality can only decidable relative to a class of display maps that does not include all diagonals [CCD17]. 3) A class of display maps can be used to present an 1-categorical structure, as in Joyal’s clans [Joy17] and Lurie’s pre-geometries [Lur09a]. The idea of identifying the relevant display maps separately from a theory and locally to a given construction was promoted and used to great effect by the exponents of the 1Pace the observation of Garner [Gar09] that the fun-split formulation is strictly stronger than the conventional formulation in the absence of the η-law — perhaps a more realistic title for the cited work would have been “On the strength of dependent non-products in the type theory of Martin-Löf”. 3 Edinburgh school of logical frameworks [HHP93; HL07]; the function of the %worlds declarations of the Twelf proof assistant [PS99] is nothing more than to specify the class of contexts relative to which a given metatheorem should hold, since almost no metatheorems hold unconditionally. (1.2·3) While the presence of higher-level hypothetical judgments in the language of locally Cartesian closed categories is convenient, it is a priori correct to worry about whether it evinces an exotic (and therefore inadequate) notion of syntax for a conventional type theory. In Section 5, we prove that the presence of hypothetical judgments of arbitrary level is a 2-conservative extension of the stratified language of Uemura’s representable map categories in the sense of Shulman [Shu20]; our result can be seen as a semantic adequacy theorem for a encodings of type theories as locally Cartesian closed categories.
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